This invention relates to the sintering of ceramic articles from a mixture of a ceramic powder with a suitable hydrocarbon binder, and is more particularly directed to a process of forming, sintering, and densifying of such articles so that they are substantially free of carbon residues.
Ceramic articles for use in electronic devices are often fabricated by mixing ceramic powder with a suitable binder, e.g. a polymeric hydrocarbon binder, molding the mixture to the desired shape, and then firing the molded article to sinter and densify the ceramic and at the same time to drive off the polymeric binder.
In the electronics industry, the structures produced by these procedures usually have conductive metallic pathways formed in them. A mixture of an alumina ceramic powder and polymeric binder powder may be combined with a pattern of a mixture of conductive powdered metal and binder, so that the ceramic substantially surrounds or encapsulates the powdered metal pattern. Because the metallization must not be oxidized, it is necessary to sinter the articles in atmospheres low in oxygen. Thus, atmospheres of gaseous hydrogen and steam are preferably employed. Attention has focused on eliminating the carbon residues that may remain from sintering in this fashion.
The conventional process for molding composites of powdered ceramics to produce sintered, dense composite articles involves a first pre-sintering heating stage to drive off any residues of the hydrocarbon polymer used to assist in molding the ceramic and metal powders. Where a low oxygen atmosphere must be employed, the carbon does not oxidize, and so incomplete removal of carbonaceous materials from the binder component is often experienced. Thereafter, when the sintering and densification heating cycle is completed, instead of a white ceramic, a black ceramic is obtained. The carbon remaining in the ceramic can impede reliable densification as well as form conduction paths which lead to lower insulation resistance and can increase dielectric losses.
The manufacture of electronic devices using ceramic powder to form an insul-ating matrix is discussed in U.S. Pat Nos. 3,808,041, 3,074,143, 3,520,054 and 4,080,414.
A recent improvement is described in Brownlow and Plaskett U.S. Pat. No. 4,474,731. That process is intended to achieve substantially complete removal of the binder and carbonaceous residue of the binder. For this purpose, a pyrolytic catalyst is included with the binder and ceramic. The pre-sintering heating step is followed by heating the ceramic and binder in a low oxygen atmosphere at a temperature in the range of 350-780 degrees celsius in the presence of the catalyst, e.g. nickel, platinum, and/or palladium ions, for a time sufficient to catalytically pyrolyze and drive off the carbon residues on the ceramic. In the Brownlow and Plaskett process, the Ni, Pt, and Pd ions are added in compound form to the polymer binder by solution prior to preparation of the ceramic powder mixtures, so there is not optimum distribution of the catalyst onto the surface of the ceramic powder particles.
A major problem is encountered in the Brownlow and Plaskett process. That is, for catalytic metals that function as heterogeneous catalysts, the catalytic metal ions should be firmly anchored on the ceramic support particles. However, the binder/metal-ion slurry used in the Brownlow and Plaskett process being chemically complex does not always place the catalyst ions on the support sites. Where the ceramic, as for example, powdered .gamma.-Al.sub.2 O.sub.3, is mixed into the binder/metal-ion slurry, and the metal-ions must be deposited onto the ceramic to perform their catalytic function during the pre-sintering, the deposition of catalyst onto the ceramic support will not occur in a predictable or reproducible manner. Consequently, the pyrolysis of carbon residues may not be complete. For example, the dilute catalyst metal concentration (typically 1.5% by weight of metal oxide based upon the dry weight of the polymer binder) will be "swamped" by the polymer which, itself, will be strongly adsorbed onto the ceramic particles. This leads to a nonuniform distribution of metal ions throughout the composite.